## Rotations

Posted on June 3, 2018

Tags: Haskell

This is just some cool-looking stuff I figure out when I was trying to figure out zipper-like algorithms. When I do get around to doing a deep dive on zippers (especially comonadic zippers) I’ll probably be able to write a full post on some of the underlying theory (with maybe some more efficient implementations).

```
{-# LANGUAGE FlexibleContexts #-}
module Rotations where
import Control.Monad.Tardis
import Control.Applicative ((<**>))
-- | >>> rotations "abcd"
-- ["abcd","bcda","cdab","dabc"]
rotations :: [a] -> [[a]]
rotations = flip evalTardis (id,id) . traverse f
where
f x = do
xs <- pure [] <**> getPast <**> getFuture
modifyBackwards ((:) x .)
modifyForwards (. (:) x)
pure xs
```